1. Field of the Invention
The present invention relates to a semiconductor laser, and an optical element with a semiconductor laser and other optical elements integrated.
2. Description of the Background Art
An emission end facet of a semiconductor laser is generally formed by cleavage, except for specific laser devices such as vertical cavity surface emitting laser devices. After cleavage, high-reflective coating, low-reflective coating, non-reflective coating, or other coatings are applied to the emission end facet, according to applications for the semiconductor laser.
There exist semiconductor laser devices which cause problems when a part of laser beam reflected at the end facet, where the laser beam emerges, returns to an active layer of the semiconductor laser device. For example, one of such laser devices is a distribution feedback laser diode (DFB-LD) which is used for optical communications, especially, laser devices called λ/4 shift DFB-LD. In this kind of laser device, a non-reflective coating is applied to the end facet. This non-reflective coating ideally means to provide a coating film with zero (0) reflectance. Although it is desirable that the reflectance should be zero, it is impossible for the reflectance to be completely zero due to variations of fabrication accuracy of the coating film, for example. Consequently, even when a non-reflective coating is provided, part of the laser beam is reflected at the end facet, in actuality.
In laser devices which require non-reflective emission end facets, a so-called window structure is frequently adopted (with respect to a principle of the window structure, see IEEE Journal of Quantum Electronics, Vol. QE-20, No. 3, pp. 236–245 (1984)). This is because, with to the window structure, a returning reflected light from the end facet can be prevented from entering into the active layer of the semiconductor laser device.
The reason why the returning reflected light can be prevented from entering into the active layer by adopting the window structure can be described as follows. Since no waveguide structure exists in a window structure section, the light entering into the window section from the active layer gradually expands and propagates. As a result, at the end facet of the laser device, a distribution of a light intensity is held expanded. Part of the light that has reached the end facet is slightly reflected even when non-reflective coating is provided. However, since the distribution of the light intensity is expanded, considerable part of the reflected light does not return to the active layer. Consequently, the reflectance can be effectively lowered by providing the window structure.
In addition, because the window structure effectively lowers the reflectance of the end facet as is the case of DFB-LD, it is also used for elements with a modulator and a laser device integrated. The window structure is further adopted for high power laser devices. In the high power laser devices, temperature rise easily occurs locally when light density is high at the end facet, and the end facet section suffers damage due to the heat and which degrades the laser device. To prevent the degradation, the window structure which gradually expands the light and lowers the light density at the end facet becomes effective.
In a conventional window structure, returning reflected light and light density can be lowered, whereas it has a problem that the outgoing beam angle is likely to deviate in the vertical direction (that is, direction perpendicular to the outgoing beam direction). The reasons are described as follows.
Semiconductor laser devices are essentially likely to have an asymmetric structure with respect to the laminating direction with the active layer set as a center. This is because the semiconductor laser devices are diodes and include p-type semiconductor and n-type semiconductor. In general, when the lower side of the active layer is p-type, the upper side is n-type, and when the lower side is n-type, the upper side is p-type. Furthermore, there are cases in which current constriction structure for concentrating current to the active layer with 1 to 2-μm-width is installed on one side of the active layer, constituting one cause of asymmetry.
In semiconductor laser devices in which InP-based and GaAs-based materials are used, not only conductivity type but also distribution of carrier concentration are likely to cause an asymmetry in vertical direction. This is because it is difficult to increase p-type carrier concentration in fabricating elements. When a difference in the carrier concentration occurs, a refractive index of the material varies due to a phenomenon called the plasma effect. As a result, the refractive index in the elements becomes asymmetrical in the vertical direction. The window section is no exception and the refractive index in the laminating direction is vertically asymmetrical with an extension of the active layer set as a center.
In semiconductor laser devices made of materials with an asymmetrical refractive index arising from the distribution of the carrier concentration, a phenomenon is observed in which an outgoing angle of the light is deviated in a perpendicular direction (laminating direction). This is because the light propagates while avoiding the area with low refractive index.
Now, a specific example where the light outgoing angle is deviated is described. In conventional laser devices, right below the optical axis of the window section (an extension of the active layer), an n-type InP layer with a high carrier concentration exists. The carrier concentration is kept high because the n-type InP layer works as a current blocking layer and the high-temperature and high-output characteristics are degraded when the carrier concentration is low. As a result, the refractive index of n-type InP layer of high carrier concentration is about 0.6% lower than that of p-type InP layer or n-type InP layer located above due to changes of the refractive index caused by the above-described plasma effect. Consequently, the outgoing light is bent upwards as it propagates in the window section, and goes out from the end facet upwards at angles of about 5 to 10°.
If the angle of the outgoing light deviates, for example, when laser outgoing light is coupled with optical fiber, problems occur such as lowered combination efficiency.